Medium, high, and extra-high voltage (“MV,” “HV,” and “EHV”) cables typically contain a peroxide cross-linked polyethylene material as an insulation layer. The incumbent process for cable manufacture involves the extrusion of the peroxide-imbibed low density polyethylene (LDPE) as the insulation layer onto a suitable conductor at a temperature where the peroxide reaction is minimal to maintain good processability, the average temperature being about 140° C. Subsequently, the cable core is conveyed through a vulcanization unit at elevated temperature, which averages about 200° C., resulting in crosslinking of the LDPE insulation material in a few minutes. After cooling, the cable core is taken up on spools for subsequent processing.
Although crosslinking provides valuable improvement in the thermomechanical properties of the material, the peroxide used for crosslinking creates byproducts that require removal from the material after it is formed into an insulation layer (e.g., by degassing) but before a jacketing layer is placed over the insulation layer. In the case of dicumyl peroxide, these byproducts include methane, acetophenone, alpha methylstyrene, and cumyl alcohol. Although work has been undertaken to discover an insulation material that does not require degassing, no viable solution has been identified to date.
A technique has been described for crosslinking polyethylene via a catalyst-aided epoxy-anhydride reaction that can reduce more than 70% of the volatile byproducts in the crosslinked material. However, achieving a suitable balance of cure rate at the disclosed 175° C. to 260° C. curing temperature and scorch retardance at 140° C. is problematic, being either considerably slower in cure rate or considerably more prone to scorch than incumbent peroxide initiated systems.
Accordingly, a need remains for a crosslinkable material suitable for use in wire and cable applications that can be produced at an acceptable cure rate at the vulcanization stage without compromising processability at extrusion stage, and requires little or no degassing after crosslinking of the material.